Aquastat hi limit controlAquastat High, Low, & DIFF vs Heating Cost / Efficiency

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Effect of Aquastat HI LO DIFF settings on heating boiler efficiency & heating costs:

This article explains how to choose the best settings for a heating boiler aquastat to obtain efficiency at the heating boiler gas or oil burner, efficiency in heat transfer into the occupied space, and minimum heating fuel costs.

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

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Heating Boiler Aquastat Control Settings Effect on Boiler Efficiency & HEating Costs

Honeywell R8182D internal (C) Honeywell Here we discuss the impact of the Aquastat settings on how long and how efficiently the boiler runs, how efficiently heat is transferred into the occupied space, and how the aquastat settings and boiler on-time are likely to affect the seasonal cost or cold-weather cost of operation and fuel.

[Click to enlarge any image]

At AQUASTAT CONTROLS and in the more detailed article AQUASTAT HI LO DIFF SETTINGS where most of these questions and answers were first posted, we explain how aquastats work, defining the functions and dials of the aquastat HI LO and DIFF control along with the reset button often found on these heating boiler controls.

Question: Isn't it less expensive to turn up the boiler temperture when heating in very cold weather such as here in Edmonton, Alberta ?

On 2017-01-12 15:10:26.520592 by Les

Our building has a radiant baseboard heating system. I live in Edmonton, Alberta Canada, and we can have some pretty harsh winter nights.

I own a suite in an 84 unit apartment. We went through a hyper freeze in early December, with wind chill factors on December 5th 2016 in the mid -30 degrees centigrade.

My suite got to 18C degrees, which is 64F and would not go any higher.

Would it not be cheaper to turn up the boiler a bit to heat the building properly, so the zone valves are not constantly calling for heat, than it is to have it turned down too low, causing a constant call for heat? The boiler nor the circulation pump should not have to work anywhere near as hard?

Could a zone valve problem cause the heat pipes in one suite to be colder than others?

If a zone valve is stuck or acting sporadically, can it be reset somehow?

If the furniture is pushed against the baseboard for only about a four foot length of the line in the room, but the vent holes on the top are clear allowing the heat to rise, how much would it affect the air circulation, and heat efficiency of the suite?

These questions and replies were posted originally at BASEBOARD HEAT and were also part of a private email exchange with the reader, January 2017.

Reply: Yes. Generally that's right.


I can but guess as I don't know anything about your building.

If a heating system is not able to maintain the building indoor temperature at the minimum required by law for rental properties where you live, it would make sense for the owners to have an experienced heating technician examine the system. I would be very reluctant to propose a "fix" before I understand the "cause" of the problem. For example, if the aquastat controlling the boiler temperature is already set to 200 degF (93.3 C) at its high limit, then it cannot be set higher without making the system unsafe.

And there could be other problems: a bad circulator pump, a pump circulating at too slow a rate in lpm through the piping, building drafts and air leaks, improper zone piping, a zone valve that's not opening when it should, a thermostat that's not properly located. Or other problems such as those discussed at

A zone valve that is stuck closed MIGHT be able to be manually latched in the OPEN position as a temporary fix.

If furnishings are blocking much of the length of a baseboard you are significantly reducing the heat output from the baseboard. Blocking ari flow into the bottom of the baseboard is as much of a mistake as blocking the warm air exit from the baseboard at its top. Blocking air inflow stops air outflow.

You can guesstimate the impact of blocking air flow through a baseboard as a percentage. The percent of blockage of length of baseboard is probably equivalent to about 80% of that percent of blockage of the heat output. I'm taking less than100 % as there is some heat output even when there is no air input.

Eg. in a 100 ft. long run of baseboard in an apartment, ten feet is blocked by furniture, curtains, etc., reducing heat output in that apartment by (.8 x 10/100) or 8 percent. - See more at:

Economizers and outdoor reset modules can also save significantly on heating costs by taking an opposite approach: setting boiler temperature down and causing the boiler to run longer.

The tradeoffs beween higher boiler temperatures and longer boiler on-time runs are discussed at OUTDOOR RESET MODULE AQUASTAT ADJUSTER

Reader follow-up:

(Jan 12, 2017) Les said:

The boiler temperature was set at 170 degrees. We had windchill factors in the -35 to 40 C range. My suite was not the only low heat complaint. In an email our caretaker mentioned that the wind hitting one corner of the building was an issue. The heat has since been raised to about 182 and the whole building is much warmer. Here is the part of the question that you missed.

Would it not be cheaper to turn up the boiler a bit to heat the building properly, so the zone valves are not constantly calling for heat, than it is to have it turned down too low, causing a constant call for heat? The boiler nor the circulation pump should not have to work anywhere near as hard? -

Since the boiler temperature was increased I visited the condo. It was minus 21C outside. I was there about 1 1/2 hours. I checked the baseboard heaters three times when I was there, and they were cold on the outside, so the zone valve had not asked for heat during my visit, but the air temperature was comfortable. That's why I ask the question above.

Moderator reply:

(mod) said:

Les I don't quite understand the question. "Run the boiler a bit?" - the boiler will run or not depending on the temperature of the hot water circulating through the heating system. The temperature of the hot water falls depending on the heat loss rate in the building. The circulator pumps are not working so hard - and have a long duty cycle. Certainly it costs less (in electricity cost) to circulate hot water than to burn fuel. We wouldn't want a boiler to run continuously except under the coldest windiest conditions.

But you're right that a too-short "on" cycle on a boiler is also inefficient - depending on the fuel. For example an oil fired heater doesn't get up to full operating efficiency until the burner has been on 3-5 minutes - maybe longer on a large commercial heating system.

Reader follow-up:

Les said:

Sorry if don't quite understand the question, so let me try again. Ours is a three year old gas fired boiler connected to old piping. My question was not "run" the boiler for a bit, it said "turn up the boiler temperature a bit". If the boiler temperature is set too low to provide adequate heat for people to heat their suites properly, they are going to increase their thermostats to try to get more heat, resulting in each of the zones calling for heat constantly.

If the boiler temperatures are increased so people can get the building, and their suites heated up properly, they turn their thermostats down to their desired temperature, the zone will no longer call for heat until it's needed. In my case, now that the boiler temp has been increased, in an hour and a half the zone did not make a call for heat, even though it was -20C outside.

To me it seems more efficient, and cheaper than having the boiler temperature set too low, and having a constant demand for hot water. So, if boiler temperature is increased by ten degrees to provide that adequate heat, is it really more expensive?
Is my question easier to understand now?

How much work is required to increase boiler temperature? Is it a matter of just turning up the temperature on the boiler, or am I right to assume it's more complicated than that?

Thanks again for your comments.

Moderator reply: why are higher Aquastat settings more efficient?

(mod) said:
Thanks Les.

Generally I Iike to set the boiler aquastat up to 180-200: hotter = more efficient. The thermal conductivity of water in finned copper baseboard is exponentially greater as the difference between heating water temperature and room temperature increases.

The best aquastat settings are discussed at "BEST AQUASTAT SETTINGS" found by searching InspectApedia for that term. See for the article.

I don't know how your building is heated, but in general yes, it's simply a matter of setting the boiler's controlling aquastat.

SOME heating systems use an add-on energy saver that lowers boiler temperature depending on the outdoor temperature - ostensibly giving longer on-cycles (more-efficient) in warmer weather. Of course using such a feature must be weighed against the system's capability of actually providing enough heat to tenants.

Optimal boiler temperature settings for efficiency (at the boiler) are a separate question from whether or not individual tenants have enough heat and a separate question about the building's overall heating efficiency.

You're right that if the baseboard temperature is too low AND if concomitantly the heat loss from a building area is too great, then the heating system may not be able to satisfy the thermostat setting. And occupants may not be comfortable.

In that case the building operator is indeed saving money on heating cost simply by not providing enough heat to get the indoor temperatures up to where the occupants want. Or perhaps not getting the indoor temperature up to where it is required by law. 68 degF is a common standard for rental properties.

In some cities such as New York, rental property laws require a minimum amount of heating (and cooling in some cases).
Excerpting from that example:

Heat must be supplied from October 1 through May 31 to tenants in multiple dwellings. If the outdoor temperature falls below 55°F between the hours of six a.m. and ten p.m., each apartment must be heated to a temperature of at least 68°F. If the outdoor temperature falls below 40°F between the hours of ten p.m. and six a.m., each apartment must be heated to a temperature of at least 55°F (Multiple Dwelling Law § 79; Multiple Residence Law § 173; NYC Admin. Code § 27-2029).

Finally, before spending more to provide more heat in living units that are too cold it would make sense to investigate why the units are too cold.

For example if there are leaky drafty windows or if walls are not insulated, it would make sense to correct those deficiencies. Otherwise we're basically heating Mother Nature. She's said, in several ways, that she's already getting more heat than is good for her - and us. (Global Warming if the alllusion isn't obvious).

Here's a tenant's rights document from the New York Attorney General

Reader follow-up: our heating baseboard has no fins, just plain copper piping

Les said:

Our piping doesn't have any fins like radiators. It's just piping covered with a baseboard cover.

If the aquastat is set at 170 but calls for heat are constant, it should not be more expensive to increase the temp to 180 and therefore reduce the call for heat.

You mentioned that the minute the water leaves the boiler there is heat loss caused by dissipation. Would that be by 2, or 5 or maybe even ten degrees?

Thanks again so much for your comments.
- See more at:

Moderator reply:


Plain copper pipes with no fins will still radiate heat but at a much lower rate.

The number of degrees of heat loss and the rate and how that loss extends across the length of a heating piping loop is going to depend on the piping lengths and location and the temperatures in the areas through which the pipes pass. So uninsulated heating pipes passing through a cold attic or crawl space lose heat faster.

While the absolute temperature reading would be incorrect (unless you mark the measurement area with flat black paint), making IR temperature measurements along a heating pipe will give reasonably accurate estimates of the temperature drop at various points along the loop.

Start right at the boiler: measure the temperature at the point where hot water leaves the boiler and measure it again where water is returning to the boiler. - See more at:

Reader follow-up:

Jan 18
I have been having a conversation with your user name (mod), but I see no way to include an attachment except by email.

Attached is both a MANUAL for CONTROL SYSTEM of our BOILER, as well as an image from the REMOTE MONITOR SHOWING BOILER TEMPERATURES On a certain day.

On the boiler temperature image it shows a target range of 170F, and referring to the manual on page 1 there is a chart showing what I believe is optimum settings for the boiler at different outdoor temperatures. My understanding from the manufacturer is that the normal range is set by a technician, and regardless of a drop in outside temperature the boiler will not exceed that setting.

In our conversation he mentioned that he likes to set his boilers at 180F to 200F for maximum heat efficiency for winter.

In our climate in Canada it is not unusual for us to experience temperatures even lower than -20F, and even colder factoring in wind chill.

As mentioned the target range for our boiler is set at 170, but when you look at the chart in the manual it shows that at -20F the boiler temperature should be at 190F.

Our service provider claims that the setting of 170F is set for efficiency and cost reduction, which makes no sense to me. How could it be more efficient if the boiler needs to run constantly whereas at 190F the boiler would not have constant demand.

How could the system possibly maintain room temperature at 170F when outside temperatures are even lower than -20F?

Your input is much appreciated.

Moderator reply:

I'll need to read and think about this a bit further but to start:

I agree and already said that higher temperatures are more efficient - especially for fuel-fired heaters as combustion is more efficient with higher operating temperatures in the combustion chamber and fewer on-off cycles. Even with an electric boiler the thermal conductivity improves at higher temperatures - just as the heat loss from the building increases in rate when there is greater temperature difference between inside and outside.

There are also practical limits to radiant floor tubing temperature depending on what floor materials are installed. Wood floors are damaged by too-hot temperatures - shrinking, opening gaps. Even a ceramic tile floor can be too hot to be comfortable for people walking in socks or barefoot.

In general, regarding your ending question, whether or not the boiler can satisfy the thermostat when it is very cold outside is not dependent on just the boiler's temperature but on the building's heat loss rate - a factor that varies enormously among buildings, and also on heating system layout, even thermostat location, and in the case of improperly-designed systems such as ones I've described, placement of the radiant heat tubing.

What am I missing or misunderstanding in your question?

Reader follow-up:

The building was built in 1979 and still has the original metal frame windows, but is not draughty. On a heat loss scale it should be about a 5 out of 10. The heating system is natural gas fired.

The factory tech support contact confirmed that higher heat is more efficient as well. In fact he said that the boiler could be run at 190F, even 200F if necessary. The night in question was -30.5C with the wind chill, and the wind direction was blasting the cold right onto my suite. That is -28.8F, and according to the manual chart, the boiler temp should have been at least 190F.

I guess my question would be, does it cost any more to run the boiler at 190F compared to 170F?

After reading the manual, I have come to the conclusion that the best temp to have the boiler at would be 190F minimum for our harsh climate. It should only need to be set once, and left alone. It turns itself off for the summer anyway, so why would the temp ever have to be changed?

Again, many thanks for your help.

Moderator reply:

I appreciate the conversation as this is an important topic and your questions help me see where I've not been clear.

Heating is more efficient, for the reasons I've stated (boiler burner efficiency and efficiency of heat transfer into occupied space) when the boiler is run at higher temperatures in cold weather. That's why I prefer 200F. That's an upper limit since higher temperatures will cause the relief valve to open, and I suspect that when experts recommend 190F it's to be on the safe side of avoiding relief valve spillage.

"More efficient" means, in concept, "less money spent on heating" - so in theory heating the building would cost a bit less with a boiler operated at higher temperatures.

However, IF when the boiler is run at lower temperatures the room thermostat in occupied space is not being satisfied that means that the building is losing heat faster than the boiler sends heat to the space, so the indoor temperature at the thermostat never reaches the set point or temperature called-for at the thermostat.

Under that condition the boiler is already operating as efficiently as it can at the burner.
There may still remain a bit of heating efficiency improvement (in heat transfer into the occupied space) at higher temperatures.

However IF the boiler was running continuously and the rooms remain below the set temperature on the thermostat, THEN when the boiler aquastat is set to a higher temperature such that the thermostat is now satisfied, just whether we've increased or decreased the net heating cost is a bit more murky.

Take an extreme example: a landlord never delivers enough heat to get indoor temperature above 55 degF in cold weather.

A rental property lawyer obtains a court order to heat the indoors to 65 degF. in those conditions.
The landlord boosts boiler operating temperature. The boiler now is able to heat the space to 65 degF in those conditions.

In that case the landlord is almost certainly spending more on heat than she was previously.

Where the analysis is more difficult are less extreme cases. If we could keep boiler "on-time" the same in two test cases, then for sure we'd save money at higher operating temperatures. When the on-time also varies then the calculation is probably possible - if we ask for help from an HVAC design engineer. She is in turn going to push back with some advice about improving building heat loss.

For the building you describe, a single pane metal framed window in a cold climate is an enormous heat pump sending heat outside in cold weather. If there are no big drafts then the windows may be the largest point of heat loss. I pose that in that case adding storm windows would be worthwhile.

Reader follow-up:

Thank you so much for your help, it makes everything much clearer. When you say you like to keep your boilers at 200F, am I right in assuming you either are a HVAC service provider, or care taker of a building? You seem to be a professional in this field, or at least know a lot about it.

Again, you help is very appreciated.

Moderator reply:

No I am not any of the things you ask. However I have operated an HVAC service and repair company, am trained on the equipment I'm discussing, and have performed those diagnosis, service, repair or installation tasks. I am and have been for more than a decade the editor/publisher of and have experience, education, training, and have performed research in/concerning topics discussed there.

As I've said elsewhere,
InspectAPedia is an independent publisher of building, environmental, and forensic inspection, diagnosis, and repair information provided free to the public - we have no business nor financial connection with any manufacturer or service provider discussed at our website.

We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone.

OPINION: In my view credentials are not enough upon which to base acceptance of much advice, on buildings, environmental hazards or other more disparate topics, since some people sport impressive credentials but might be biased or misinformed about something on which they're expressing a vehement opinion. Or they may have conflicting interests that are not disclosed.

It's important to form some understanding of the material for one's self and to test its coherence as well as to look for its authoritative sources, references, citations, supporting research. Nearly always we cite an authoritative source, cite supporting research, or where appropriate, we tag text with OPINION if that's what it is.

You're right-fairly about credentials and expertise. You're welcome to see the editor's resume

On 2016-12-11 15:16:17.752995 by temps - disagrees with efficiency opinions

The "efficiency" part of this website is wrong. The higher the temperature of the boiler, the more heat is lost through the insulation.

Decades ago, people believed that running an engine was more efficient than shutting if off and starting it back up when needed, they also believed that leaving the lights on all the time was more efficient than turning them off when leaving the room. ALL has been proven wrong..... the truth is that the break even point for a lite bulb is around 30 seconds.

In other words, if you are leaving the room for more than 30 seconds, you WILL save energy by turning the lights off! In this article, the writer talks about the "theoretical" computation that might be needed, and completely leaves out the #1 most important factor, that heat is lost through a boiler 24 hours per day through the insulation and the ONLY way to cut that back is to lower the water temperature in the unit, or keep if off completely! So, this article is terribly flawed when it makes the claim that you might save energy by keeping the water warm all the time vs. shutting the unit off for most of the day.

OK, so it's true that gaskets will shrink and there can be leaks as the boiler cools to room temperature, but that's a different conversation and it NOT an excuse to ignore physics. Consider this: if a boiler is shut down completely and the water cools to room temperature, when it's finally turned back on, it will probably run for a continuous 1/2 hour and on an average residential unit use a 1/2 gallon of oil.

If the unit was allowed to keep the water hot all day, it would occasionally run for 5 minutes between the high and low settings of the aquastat, and do this 20 times per day. That 125 (5x20) of oil consumption is far far greater than the 1/2 our it would take to get the boiler back up to full heat after a full cooling to room temperature.

On 2016-12-11 17:35:18.499029 by (mod) - opinions about aquastat settings and heating efficiency or heating costs

Sorry temps but you are not quite correct and some of your charges are mistaken, perhaps by some simple misunderstandings about when heat is or should be on or off.

I agree that turning off the lights in an unoccupied room saves energy, moreso when incandescent bulbs are installed.

Nowhere do I suggest that we should "keep the water [in the heating baseboards, radiators, boilers] all the time - as you charge. I do argue that as long as the thermostat is calling for heat, the longer the burner is on and the higher the temperature to which it runs, the more efficient is the operation of the heating system.

Heating boilers or furnaces will operate more efficiently at higher heater temperatures. That has to do with both the combustion efficiency and the heat transfer into the occupied space.

As a separate point, you are correct that the rate of heat loss from a building is greater when there's a greater temperature difference between inside and outside.

By no means does that indicate that we should run our heating equipment at a lower temperature. Doing so would in fact not make the building occupants more comfortable.

Rather, the correction for a poorly insulated building is to improve the building's insulation, not to run the heat at a lower temperature leaving occupants cold.

OPINION: It seems a virtual certainty that in a typical heating boiler installation, heat loss and thus energy loss through the boiler itself, through the boiler's insulated jacket and into the building, is trivial compared with the heat loss from the whole building to the outdoors.

Reader comment: Ben disagrees and discusses Delta-T

Ben said:

So there is quite a bit of incorrect information in this article.

First, running a high boiler temp will increase the heat output of the fin/tube heaters in the room, HOWEVER, this significantly reduces the efficiency of the boil itself since the Delta T at the boiler is less. The goal is to run lower water temps and have longer burn times to reduce heating oil consumption. Outdoor reset is a good example of this. Unless you had a poorly calculated heat loss done when installing your heaters, there shouldn't be a reason to go up to 200*F.

Second, there is no 100% set rule for the low limit. This setting is only for when the thermostat isn't calling for heat (what the boiler will maintain during the summer is a way to think about it). if you have tankless hot water then this controls your hot water temp when not in heat mode. More than likely though this can remain very low since the BTUs required for a shower or a dishwasher is much lower than a whole house heating loop (which is what your boiler is sized for).

Finally Diff setting. This article makes it sound like the diff setting only works off the low switch...which isn't true. The Diff setting sets the upper limit of the high/low settings. say you're running 180/160 with a diff of 10....this would allow 190/170. Now keep in mind, it has nothing to do with the diff under the setpoints. This saying, the boiler will always maintain a 10* below set point. Running a high Diff can increase burn times, but also create variation in water temp....this generally is considered "uncomfortable" by most people (rad temps varying or shower temp not always similar).

So bottom line.....for fuel savings, set the boiler to have the largest Delta T at the BOILER not at the fin/tube.

Reply: use an anti-scald valve when making domestic hot water for washing and bathing

Ben, we agree and have stated that longer boiler-burner ON times are more efficient ways to run any oil fired heating system.

Our comments AND illustration of how the DIFF works on aquastats is excerpted direclty from the manufacturer's control installation manual data sheets [Honeywell's engineers in this case].

About water temperatures and uncomfortable operation, indeed we, and most manufacturers, recoimmend use of an automatic tempering valve at the boiler where a tankless coil is in use, both to avoid scalding burns and to permit higher boiler temperature operation for both greater efficiency and more total hot water supply (longer hot water at the shower before running out).


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